This invention relates to a process for preparation of alkylated diphenylamine that is liquid at room temperature. More specifically, this invention relates to a process for alkylation of diphenylamine (DPA) with polyisobutylene (PIB) oligomers having a molecular weight in the range of 120 to 600 and having a methylvinylidene content of at least 25%. Preferably, the reaction is carried out in the presence of a clay catalyst, but other types of catalysts can be used. The alkylated diphenylamine prepared by the disclosed process exhibits excellent antioxidant characteristics in lubricating oil formulations.
Alkylated diarylamines, such as alkylated diphenylamine, are well known in the art to be effective stabilizers or antioxidants in a wide variety of organic materials, including, among other organic materials, mineral oil derived lubricants and synthetic lubricants. In this use, light colored, liquid (at about 20xc2x0 C.) products are desirable for a number of practical reasons.
Alkylation of diarylamines, such as diphenylamine, with olefins in the presence of suitable alkylation catalysts is well known in the art. For example, U.S. Pat. No. 2,943,112 (Popoff et al.) teaches a two step process whereby alkylation of diphenylamine with relatively unreactive olefins, such as secondary alkenes (column 4, line 9-23), is followed by an alkylation reaction with more reactive olefins to scavenge the unreacted diphenylamine. Popoff also teaches the use of acid activated clay as an alkylation reaction catalyst to achieve the desired light color. (See also, U.S. Pat. No. 3,452,056 wherein acid clay, AlCl3 and ZnCl2 are mentioned as suitable catalysts).
Similarly, Franklin, U.S. Pat. No. 4,824,601, (column 1, lines 26-67), teaches the use of acidic clay catalysts to alkylate diphenylamine and further teaches that a light colored, liquid product may be prepared by process comprising reacting the alkylation reactants within certain molar ratios and temperature ranges for a time sufficient to ensure the alkylated product contains less than 25% dialkylated diphenylamine. This low proportion of dialkylated diphenylamine is disclosed as necessary to avoid the formation of crystallized, solid products, which are not advantageous in terms of ease of handling, transportation, storage and incorporation into the substrate to be stabilized.
More recently, addressing the same problem; that is, preparing an effective antioxidant from diphenylamine that is liquid at room temperature, Lai in U.S. Pat. Nos. 5,672,752 (Lai 1) and 5,750,787 (Lai 2), teaches processes for alkylating diphenylamine with linear alpha olefins (Lai 1) and diisobutylene (Lai 2) in the presence of a clay catalyst. These processes, as disclosed, selectively result in a higher proportion of monoalkylated diphenylamine and a lower proportion of unsubstituted diphenylamine and/or disubstituted or polysubstituted diphenylamines. These patents further disclose that to obtain the desired liquid product, the ratio of olefin to diphenylamine in the reaction mixture, together with reaction temperature and time is important to give a product mixture with less than 25% dioctyldiphenylamine, less than 25% unreacted diphenylamine and greater than 50% by weight monooctyldiphenylamine based on the total weight of the diphenylamine and alkylated DPA.
In U.S. Pat. No. 6,204,412 (Lai 3) Lai discloses yet another method of alkylating diphenylamine to obtain a light colored, liquid product, which comprises a two step method wherein, in the second step, a second olefin is added to the reaction mixture containing diphenylamine and diisobutylene (and/or an alpha-olefin of the disclosed formula) to scavenge or reduce the amount of unreacted diphenylamine in the product As with Lai 1 and Lai 2, specific mole ratio ranges, reaction temperatures and reaction times are disclosed as important to obtain the desired alkylated diphenylamine that is liquid at room temperature.
The present invention relates to the preparation of alkylated diphenylamine and other alkylated aromatic amine antioxidant compositions using highly reactive polyisobutylenes as the alkylating reagent. Highly reactive polyisobutylenes (HR PIB) are valued for use in the lubricating oil additive industry and are commercially produced in the chemical industry. These valued HR PIB polymers are characterized by the presence of significant amounts of 2-methylvinylidene isomers, R(CH3)Cxe2x95x90CH2 where R is a PIB residue, and a molecular weight preferably in the range of about 500 to 5000. Such 2-methylvinylidene polyisobutylenes are typically prepared using boron trifluoride catalysis. The preparation of such polyisobutylenes in which the methylvinylidene isomer comprises a high percentage of the total olefin composition is described in U.S. Pat. Nos. 4,152,499 and 4,605,808, the disclosures of which are incorporated herein by reference.
It is generally known by those skilled in the art that the commercial HR PIB production process described above generates a HR PIB distillate byproduct that; (i) contains mostly C8H16 to C28H56 oligomers (average C14H28), (ii) exhibits a relatively low average molecular weight of about 200 and, (iii) contains 2-methylvinylidene in the amount of at least about 25%. This low molecular weight distillate byproduct, which amounts to several percent of total HR PIB production, has little value and is typically sold as a waste product. Thus, a need exists in the chemical industry to develop a valuable use for the HR PIB distillate byproduct. It is the object of the present invention to meet this need by providing a novel process for preparation of a valuable alkylated diphenylamine antioxidant that is liquid at room temperature (about 20xc2x0 C.) using the HR PIB distillate byproduct, or fractions thereof. For purposes of this invention, the term xe2x80x9cpolyisobutylenexe2x80x9d refers to the HR PIB distillate byproduct and is further defined as comprising a mixture of highly reactive polyisobutylene oligomers, said mixture containing mostly C8H16 to C28H56 oligomers (average C14H28), exhibiting a molecular weight in the range of about 120 to about 600 (an average of about 200), containing a 2-methylvinylidene amount of at least about 25% and containing diisobutylene (C8) in a fractional amount ranging from 0% to about 50%.
There are several advantages to using the presently employed polyisobutylene oligomers to alkylate diphenylamine and other aromatic amines, including, for example: (1) the alkylation reaction can be carried out at elevated temperatures under low pressure; (2) flexibility in utilizing a new, previously wasted feedstock, which can be comprised of specific carbon number fractions, a mixture of selected fractions or, most desirably, use of the total, unfractionated HR PIB distillate byproduct, with or without removal of inert solvent (2% to 10%); (3) the alkylation reaction can be carried out at relatively lower temperatures, which because of the greater reactivity afforded by the presence of 2-methylvinylidene results in a liquid alkylated product and no solid product as is usual in low temperature reactions with conventional diisobutylene, and; (4) milder, low temperature reaction conditions can be employed to control fragmentation.
Accordingly, the present invention provides a novel, one-step, process for the preparation of alkylated diphenylamine, which comprises reacting diphenylamine with polyisobutylene in the presence of a suitable alkylation catalyst at a suitable alkylation reaction temperature less than about 200xc2x0 C., wherein said polyisobutylene contains at least about 25% of a methylvinylidene isomer and wherein the number average molecular weight of said polyisobutylene is in the range of about 120 to about 600 and wherein the mole ratio of said polyisobutylene to said diphenylamine is in the range of 1.0:1.0 to 4.0:1.0 and forming an alkylated diphenylamine product wherein said alkylated diphenylamine product is liquid at room temperature and atmospheric pressure.
In addition to diphenylamine, other aromatic amines are subject to alkylation by the disclosed process. Such other aromatic amines include, for example: N-phenyl-1-naphthylamine, N-phenyl-2-naphthylamine, p,pxe2x80x2-phenylenediamine, phenothiazine, phenoxazine, p-amino-diphenylamine, p-methylamino-diphenylamine and p-isopropylamino-diphenylamine.
As noted above, the present invention provides a liquid alkylated diphenylamine, which is an effective antioxidant, by a process comprising alkylating diphenylamine with a polyisobutylene oligomer under suitable alkylation reaction conditions in the presence of a suitable alkylation catalyst. Generally, the polyisobutylene will have a number average molecular weight of about 120 to about 600, preferably in the range of about 150 to about 400, and more preferably in the range of about 160 to about 280 and a methylvinylidene content of at least about 25%, preferably in the range of about 25% to about 95%, more preferably in the range of about 40% to about 95%, and even more preferably in the range of about 60% to about 90%. Diisobutylene (C8) may be present in the polyisobutylene in a fractional amount ranging from 0% to about 50% and preferably from about 10% to about 50%.
The use of clay as catalyst in the alkylation of diphenylamine is disclosed in U.S. Pat. No. 3,452,056, which describes the alkylation of diphenylamine with alpha-methylstyrene and related olefins with clay as the catalyst. In U.S. Pat. No. 2,943,112 and other prior art, clay is described as having several advantages including, for example: (1) as having the advantage of giving lighter colored product, (2) ease of removal by filtration after the reaction and (3) a lower degree of yellow color in the alkylated product. As a catalyst, clay and other Lewis Acids, such as AlCl3 or BF3 are generally taught as being interchangeable.(See, U.S. Pat. Nos. 3,452,056 and 5,672,752 at col. 3, In. 3-6 and col. 1, In. 33-41, respectively). More recently, U.S. Pat. Nos. 5,672,752; 5,750,787 and 6,204,412 identify certain commercially available clay catalysts, including; Filtrol(trademark) and Retrol(trademark) available from Engelhard; Fulcat(trademark) 14, Fulmont(trademark) 700C, Fulmont(trademark) 237, and Fulcat(trademark) 22B available from Laporte Industries; and Katalysator(trademark) K10 available from Sud-Chemi. These clays may include acid activated or acid leached clays. The clay catalysts may contain some water as received. Removal of the water prior to use results in a lighter colored reaction product. Therefore, it is desirable to use clay with low water content or remove the water by heating the clay with a nitrogen sweep or with vacuum stripping. Acid activated clays are preferred; however, Lewis Acids such as AlCl3 or BF3, and BF3 complexes of diethyl ether, phenol, including mixtures thereof with clay could be used as well. For purposes of this invention the preferred catalyst was determined to be Engelhard F-24 acid-activated clay (formerly Filtrol""s Retrol clays).
Generally the reaction temperature for the alkylation reaction will be in range of about 125xc2x0 C. to about 195xc2x0 C., preferably in the range of about 135xc2x0 C. to about 175xc2x0 C., and more preferably in the range of about 155xc2x0 C. to about 165xc2x0 C. When using homogeneous acid catalysts, a lower temperature such as 75-100xc2x0 C. may be used. The reaction can be carried out at a single temperature or, sequentially, at different temperatures.
The reaction pressure can range up to about 250 psi or higher, preferably below about 250 psi, and more preferably in the range of about 25 to 100 psi. Inert gas, such as nitrogen, can be used to minimize oxidation of products during reaction, but mostly to allow operation at higher temperatures with the lower boiling oligomers. A nitrogen or other inert gas atmosphere, in contrast to air, suppresses the formation of products that deactivate the clay catalyst. Other benefits of nitrogen pressure include higher rates of reaction, shorter reaction times, and enhanced formation of dialkylated DPA. The latter was achieved by preventing the loss of volatiles to the atmosphere.
The molar ratio of polyisobutylene to diphenylamine is normally in the range of about 1.0:1.0 to 4.0:1.0, preferably in the range of about 1.25:1.0 to 3.0:1.0, and more preferably in the range of about 1.5:1.0 to 2.8:1.0. Tables 1-3 in the Examples set forth the mole ratios of polyisobutylene to diphenylamine chosen for Examples 1-18. Other mole ratios may be chosen. The mole ratio chosen for the reaction will affect the degree of DPA conversion to alkylate. When diisobutylene is present in the polyisobutylene, preferably in the range of about 10 percent to about 50 percent, the mole ratio of diisobutylene:diphenylamine is less that about 0.6:1, preferably less than about 0.55:1 and more preferably less than about 0.45:1.
In practice it is difficult to take the reaction to the extinction of DPA because as the reaction continues the concentration of reactants diminishes and the rates become very slow. Generally, less than 5%, and more typically less than 1% DPA, is left unreacted in the product mixture. If desired, lower residual amounts of diphenylamine can be obtained by continued stripping or, alternatively, by secondary treatment with a more reactive olefin such as isobutylene or styrene. Preferably, the polyisobutylene should be incrementally added to the reaction mixture to control and maintain the desired reaction temperature and to minimize premature PIB isomerization to non-reactive olefins. However, depending on the desired product mixture it may be beneficial to add HR PIB oligomer to the reaction mixture all at once at a lower temperature than the reaction temperature.
If desired, the reaction can be carried out in a neutral solvent such as mineral oil or an inert hydrocarbon solvent, but generally no solvent is necessary.
Reaction time is a very flexible reaction parameter and is dependent on the reaction temperature, mole ratio of reactants and catalysts and pressure. Generally, the reaction will be carried out over a period of about 2 to 30 or more hours, preferably over a period of about 5 to 24 hours, and more preferably over a period of about 6 to 16 hours.
Upon completion of the reaction, the desired alkylated diphenylamine products can be isolated using conventional techniques, such as stripping under vacuum or separation by elution with hexane using column chromatography. Depending upon the particular reaction conditions selected the amount of monoalkylated DPA to dialkylated DPA in the product mixture can typically range from 34 to 68 mole % for the monoalkylate to 32 to 66 mole % for the dialkylate; however, under certain conditions the product mixture can be either predominantly monoalkylate or predominantly dialkylate. For example, in Example No. 13 shown in Table 2 below, the molar % ratio of monoalkylate to dialkylate was 44 to 56; in Example No. 3 shown in Table 1, below, the molar % ratio was 85 percent mono isomer and 15 percent dialkylated isomer; and in Example No. 1 shown in Table 1 the product contained exclusively mono isomer and a small amount of unreacted DPA. In all Examples 1-18 the alkylated products were liquid at room temperature (about 20xc2x0 C.) and ambient pressure.